mushroom,the conspicuous umbrella-shaped fruiting body (sporophore) of certain fungi, typically of the order Agaricales (q.v.) in the phylum Basidiomycota (q.v.) but also of some other groups. Popularly, the term mushroomis used to identify the edible sporophores; the term toadstoolis often reserved for inedible or poisonous sporophores. There is, however, no scientific distinction between the two names, and either can be properly applied to any fleshy fungus fruiting structure. In a very restricted sense, mushroomindicates the common edible fungus of fields and meadows (Agaricus campestris). A very closely related species, A. bisporus, is the mushroom grown commercially and seen in markets.

Umbrella-shaped sporophores are found chiefly in the agaric family (Agaricaceae), members of which bear thin, bladelike gills on the undersurface of the cap from which the spores are shed. The sporophore of an agaric consists of a cap (pileus) and a stalk (stipe). The sporophore emerges from an extensive underground network of threadlike strands (mycelium). An example of an agaric is the honey mushroom (Armillaria mellea). Mushroom mycelia may live hundreds of years or die in a few months, depending on the available food supply. As long as nourishment is available and temperature and moisture are suitable, a mycelium will produce a new crop of sporophores each year during its fruiting season.

Fruiting bodies of some mushrooms occur in arcs or rings called fairy rings. The mycelium starts from a spore falling in a favourable spot and producing strands (hyphae) that grow out in all directions, eventually forming a circular mat of underground hyphal threads. Fruiting bodies, produced near the edge of this mat, may widen the ring for hundreds of years.

A few mushrooms belong to the order Boletales(q.v.), which bear pores in an easily detachable layer on the underside of the cap. The agarics and boletes include most of the forms known as mushrooms. Other groups of fungi, however, are considered to be mushrooms, at least by laymen. Among these are the hydnums or hedgehog mushrooms, which have teeth, spines, or warts on the undersurface of the cap (e.g., Dentinum repandum, Hydnum imbricatum) or at the ends of branches (e.g., Hydnum H. coralloides, Hericium caput-ursi). The polypores, shelf fungi, or bracket fungi (order Polyporales; q.v.) have tubes under the cap as in the boletes, but they are not in an easily separable layer. Polypores usually grow on living or dead trees, sometimes as destructive pests. Many of them renew growth each year and thus produce annual growth layers by which their age can be estimated. Examples include the dryad’s saddle (Polyporus squamosus), the beefsteak fungus (Fistulina hepatica), the sulfur fungus (Polyporus P. sulphureus), the artist’s fungus (Ganoderma applanatum, or Fomes applanatus), and species of the genera Fomes and genus Trametes. The clavarias, or club fungi (e.g., Clavaria, Ramaria), are shrublike, clublike, or coral-like in growth habit. One club fungus, the cauliflower fungus (Sparassis crispa), has flattened clustered branches that lie close together, giving the appearance of the vegetable cauliflower. The cantharelloid fungi (Cantharellus and its relatives) are club-, cone-, or trumpet-shaped mushroom-like mushroomlike forms with an expanded top bearing coarsely folded ridges along the underside and descending along the stalk. Examples include the highly prized edible chanterelle (C. cibarius) and the horn-of-plenty mushroom (Craterellus cornucopioides). Puffballs (family Lycoperdaceae; q.v.), stinkhorns(q.v.), earthstars (a kind of puffball), and bird’s nest fungi are usually treated with the mushrooms. The morels (Morchella, Verpa) and false morels or lorchels (Gyromitra, Helvella) of the phylum Ascomycota (q.v.) are popularly included with the true mushrooms because of their shape and fleshy structure; they resemble a deeply folded or pitted conelike sponge at the top of a hollow stem. Some are among the most highly prized edible fungi (e.g., Morchella esculenta). Another group of ascomycetes includes the cup fungi(see cup fungus), with a cuplike or dishlike fruiting structure, sometimes highly coloured.

Other unusual forms, not closely related to the true mushrooms , but often included with them, are the jelly fungi (Tremella species), the ear fungus or Jew’s ear (Auricularia auriculara-judae), and the edible truffle(q.v.).

Mushrooms are free of cholesterol and contain small amounts of essential amino acids and B vitamins. However, their chief worth is as a specialty food of delicate, subtle flavour and agreeable texture. By fresh weight, the common commercially grown mushroom is more than 90 percent water, less than 3 percent protein, less than 5 percent carbohydrate, less than 1 percent fat, and about 1 percent mineral salts and vitamins.

Poisoning by wild mushrooms is common and may be fatal or produce merely mild gastrointestinal disturbance or slight allergic reaction. It is important that every mushroom intended for eating be accurately identified (see mushroom poisoning).